The default command adding a 'ruby' is below. To create your own gem go to https://www.khanacademy.org/computer-programming/minecraft-gem-generator/5588266993975296
Easy gem creator for Vanilla Minecraft. Easily a add custom gem to minecraft!
The ore for your gem will look exactly like chiseled stone bricks. This will camoflauge into caves!
It will spawn in dark caves and anywhere dark for that matter. It could spawn right infront of your eyes! By making a very dark room you could 'farm' your ore
(it really just replaces any bats with your custom ore, so spawn a bat to cheat it in)